Measurement of the echo reduction for underwater acoustic passive materials with a time reversal technique

A method of measuring the echo reduction of passive materials with the time reversal (TR) technique is presented. To measure the echo reduction with this approach, the received signals are firstly focused according to the TR theory with a sample. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal. Finally, the echo reduction of the sample is evaluated with these focusing signMs. Besides, to calibrate the echo reduction measured via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 m×l.0 m×0.005 m are tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results of the free field for the measured frequency range of 0.5~20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dE. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries or low frequency sources.